The threat of global warming has caused increasing alarm, both scientific and public, over the past two decades. Although debate is ongoing, a number of leading environmental scientists believe that global warming is a result of increased level of carbon dioxide and other greenhouse gases in the atmosphere. Studies have shown that atmospheric carbon dioxide levels in the late 1950s were around 315 ppm and have been rising ever since. Recent data has shown that the carbon-dioxide levels in the atmosphere had risen to about 376 ppm at the end of 2003.
The increased levels of atmospheric carbon dioxide have been attributed largely to increased burning of fossil fuels.
As a result of growing concern over the potentially devastating effects of global warming, the United Nations Framework Convention on Climate Change (UNFCCC) was adopted in 1992. By June 1993, the convention had received signatures from 166 countries. The Kyoto protocol, which is a protocol to the UNFCCC, was adopted at the third session of the conference of the parties to the UNFCCC in Kyoto, Japan, on 11 Dec. 1997. The provisions of the Kyoto protocol attempt to regulate the output of carbon dioxide by member states that have signed and ratified the protocol. Although a large number of countries have agreed to be bound by the provisions of the Kyoto protocol, neither Australia nor the United States of America are yet to ratify the protocol.
The levels of atmospheric carbon dioxide are controlled by two factors, these being (a) the amount of carbon dioxide being emitted into the atmosphere and (b) the amount of carbon dioxide being removed from the atmosphere into carbon sinks. Carbon sinks act as reservoirs for storing carbon dioxide. Carbon sinks may include biomass (such as forest and crops), plankton, soils, water bodies and geosequestration sinks. Thus, the net carbon dioxide emissions of any particular country are calculated by determining total carbon dioxide emissions and total carbon dioxide taken up by carbon sinks.
The UNFCCC allows for a system of carbon trading. Under this system, parties who establish carbon sinks obtain a “carbon credit” in respect of the amount of carbon dioxide taken up into the carbon sinks. This carbon credit can be traded to greenhouse gas emitters in order to enable the emitters to meet their targets under the Kyoto protocol.
It is known that as a result of plant growth, carbon based materials form in a plant structure and the carbon based materials may return to the soil to form carbon-containing deposits. There is established methodology to determine soil organic carbon content, which is a factor of interest to those managing agricultural businesses. Most organic material that is returned to the soil eventually rots and its carbon content is emitted over a period of time as that material rots away.
Phytoliths, also referred to as plant opal, are silica features that form in plants as a result of biomineralisation. Silica that occurs within soils is taken up by the root system of a plant in the form of silicic acid (SiOH4), and subsequently deposited throughout the intra cellular and extra cellular structures of their leaf, stem and root systems. Piperno (1988), Phytolith Analysis: “An Archaeological and Geological Perspective” (Academic Press London) describes three sites of silica deposition within plant tissue. The sites comprise (1) the cell wall deposits, often called membrane silicification, (2) infillings of the cell lumen, and (3) in intercellular spaces of the cortex. The cell wall deposits often replicate the morphology of the living cells, while those forming in the lumen do not.
The presence of carbon within phytoliths was first discussed by the Australian CSIRO scientists Jones and Milne (1963) “Studies of Silica in the Oat Plant”, Plant and Soil XVIII(2):207 220. Following this initial research, a number of studies on carbon in fossil phytoliths were undertaken. Such studies have concentrated on radiocarbon dating of fossil phytoliths to establish stratigraphic chronologies for archaeological and palaeobotanical research or δ13C isotope values to determine palaeovegetation types based on C3 and C4 signatures. The presence of organic carbon in phytoliths has been recognised for its role in radiocarbon dating. Radio carbon dating has been conducted by the inventors and it has been demonstrated that the carbon occluded within phytoliths in a soil depth of up to around 1.2 m was in the order of 8,000 years old. The inventors' studies do not explore to a limit position sequestration of carbon in phytoliths but there is no evidence to suggest that there would be appreciable carbon release over much longer periods of phytolithic storage under most soil conditions.
It is also known that phytoliths can be separated from other organic fractions by heavy liquid flotation (for example, using a specific gravity of 2.35 gcm−3) or, as reported by one of the present inventors (i.e. Parr), acid digestion of the organic and carbonate component, thus leaving a silica residue (see one of the inventor's (i.e. Parr) publication “A composition of heavy liquid floatation and microwave digestion techniques for the extraction of fossil phytoliths from sediments.” Review of Palaeobotany & Palynology, 120 (2002): 315-336).